Proposed research will attempt to develop a model which promotes a better understanding of how the size distribution of cities in the United States changes in response to economic forces. The city size distribution in recent years has been altered markedly in favor of cities of the largest size class--metropolitan areas of three million and over. Some have conjectured that by the end of the century as high as 95 percent of the U.S. population will live in 15 or so large city regions. The basic hypothesis is that this change in the distribution of city sizes is a direct and measurable consequence of the drawing power of net economies of agglomeration and scale which are present and more strongly manifest in cities of this size than of the other sizes. Multiple regression techniques will be used to test this hypothesis. If the hypothesis is verified the policy implications may be of interest. One implication might be that a national population policy should encourage rather than discourage the growth of the largest city regions. If for some reason the predicted future distribution of city sizes is deemed undesirable, then scenario analysis might be applied to the model to see what present policies should be pursued to reach an alternative future. In addition to presenting a model of a changing city size distribution, the study will look into the possibilities for scenario analysis.